1. Field of the Invention
The present invention relates to an optical color separation system provided in a high resolution color image pickup device, such as a still video camera or a color copier, and so forth, to prepare separate color components of an optical image of an object. The present invention also relates to a single lens reflex type of optical color separation system.
2. Description of the Related Art
In a known high resolution image pickup device, images of three primary colors, i.e., red, green and blue (R, G, B) are formed on a monochromatic recording medium, and are combined in an optical or electrical process to obtain a colored picture. In general, a known optical color separation system to prepare three primary color image components is comprised of a single prism or a plurality of prisms.
For example, an optical system disclosed in Japanese Unexamined Patent Publication No. 1-319384 is comprised of a dichroic prism and two light path forming prisms which are arranged symmetrically with respect to the optical axis of light incident upon the dichroic prism. The dichroic prism includes dichroic mirror surfaces (equivalent to dichroic mirrors) which are perpendicular to each other to reflect a red component and a blue component, respectively to separate the incident light into three primary color components R, G and B. The light path forming prisms are each provided with a total reflection surface which reflects a corresponding light component (e.g., the red or blue component) transmitted through the dichroic prism. In this optical system, a green component of the light incident upon the dichroic prism is transmitted through the two dichroic mirrors and is converged onto an image forming surface at a green image forming position thereof. The red and blue components are reflected by the respective dichroic mirrors in directions perpendicular to the optical axis of the incident light and different from one another by 180.degree. and are thereafter reflected in a direction parallel with the optical axis due to the total reflection by the light path forming prisms. The red and blue components thus reflected pass through the corresponding light path forming prisms in a direction parallel with the optical axis of the green component. Consequently, the optical path length increases, so that the red and blue components are converged onto the image forming surface at green and red image forming positions in proximity to the green image forming position, respectively.
In Japanese Unexamined Patent Publication No. 6-167603, a three primary color separation prism is comprised of a dichroic prism and two light path forming prisms integral with the dichroic prism, which have been disclosed in Japanese Unexamined Patent Publication No. 1-319384. The color separation prism is provided therein with two total reflection surfaces to realize an identical optical path length of the color components. In other words, the green component transmitted through the two dichroic mirrors is reflected by one of the total reflection surfaces in a direction perpendicular to the optical axis of the incident light and different from the directions of reflection of the red and blue components by 90.degree., and is thereafter reflected by the other total reflection surface in a direction parallel with the optical axis. As a result of an increase in the optical path length of the green component, the green, red and blue images formed on the image forming surface are not aligned along a line but are arranged in a zig-zag fashion.
In the known optical system, since the optical path length within the prism is intentionally increased to adjust the optical path lengths of the color components and image forming positions thereof, the optical path length within the glass medium is increased, and the incident light is refracted. Consequently, there is a large spherical aberration caused, thus resulting in a difficulty in forming a high quality image. Moreover, if the optical path length of the green component is elongated, as in JP 6-167603 mentioned above, the green image forming position on the image forming surface is not aligned with the red image forming position or the blue image forming position, and hence, for example, the scanning operation of the image is complicated.
Also, in the conventional optical system as mentioned above, all the reflection surfaces including the dichroic mirror surface adapted to prepare separate color components and the total reflection surfaces adapted to provide the optical path for light reflected thereby are formed in the prism, the image forming positions vary depending on the machining precision of the prism. To this end, the prism must be machined at an extremely high precision, which however increases the manufacturing cost.